Thermo-optic effect generally represents the change in optical properties of a material due to heat radiation. This phenomenon can be exploited in various beneficial ways. For instance, a common element used in optical waveguide applications is a thermo-optic device.
In such a device, a portion of a waveguide is heated using of a local resistive heating element. The heat generated by the heating element causes the optical signal within the waveguide to phase shift in accordance with the thermo-optic effect. The induced phase shift can be represented by the change of refractive index with change in temperature (i.e., dn/dT). Additional heat induced phase shift may further result from thermal expansion of waveguide. The waveguide output remains at a fixed wavelength if temperature is constant. The resistive heaters are fabricated by the deposition and patterning of metal films (e.g., aluminum, tungsten, nickel, chrome, ni-chrome, gold, or platinum) or semiconductor materials (e.g., polysilicon). Other conventional heater designs include localized Peltier elements.
In operation, a predetermined amount of power is applied to the heater of the thermo-optic device, with that predetermined power being correlated to a desired heater temperature, which is in turn correlated to a desired phase response in the optical waveguide. Numerous techniques are available for applying the desired amount of power, ranging from simple switching circuits that deliver pre-calibrated power levels to more complex feedback circuits that adjust power delivered to the heater element in real-time as the local temperature and/or waveguide output wavelength are monitored. Sensors (e.g., temp, wavelength, etc) are typically used in such feedback systems
In any case, conventional thermo-optic heaters are associated with a number of problems. For instance, large metal film and polysilicon heaters are required to be a non-trivial distance from the waveguide, in that such heaters generally have a large thermal profile and very high power consumption. Positioning the heaters too close to the waveguide impedes the effective index variation. In addition, fabricating such conventional heater elements requires additional semiconductor process steps (e.g., deposition, masking, etching, etc). Moreover, these added steps increase the opportunity for manufacturing loss and lower yields.
What is needed, therefore, are new thermo-optic heater designs. In a more general sense, there is a need for devices that can be used to maintain a desired temperature at a semiconductor device.